Portable devices such as smartphones, tablets, console games or e-book readers, usually utilise multiple supply voltages to power up different internal modules.
Some applications require bipolar power supplies with different regulated voltages, meaning that both positive voltage and negative voltage are needed.
Audio module and display panel are some of examples compliant with the foregoing.
For instance, Active Matrix Organic Light Emitting Diodes (AMOLED) display panel, which is a good candidate for portable devices due to its high display quality (i.e. brightness, contrast, vividness and wide viewing angle) and low power consumption requires the supply of both a positive voltage and a negative voltage to operate properly.
A power converter such as a Single Inductor Bipolar Output (SIBO) converter may be used to provide such positive and a negative power supply.
However, most of SIBO converters are exclusively operated in a boost type (SIBO-Boost) configuration, meaning that an input voltage can only be stepped-up to produce a higher positive output voltage, or in a buck type (SIBO-Buck) configuration, meaning that an input voltage can only be stepped-down to produce a lower positive output voltage, but not both.
This is inappropriate for newest technologies of Organic Light Emitting Diodes (OLED), which requires a positive supply voltage that can be either higher or lower than the classical Li-Ion battery voltage and thus requires a voltage converter able to operate either in Buck or Boost mode.
Therefore, an appropriate SIBO converter should be able to operate both as a SIBO-buck and as a SIBO-boost in a controllable manner, thus corresponding to a SIBO-Buck-Boost (SIBOBB), for both a positive output voltage and a negative output voltage. In this case, it would be possible to step-up or step-down an input voltage when needed to produce a positive output voltage or a negative output voltage.
A converter addressing that kind of requirements is proposed in document (1): WEI-CHUNG, Chen, et al. A single-inductor bipolar-output converter with 5 mV positive voltage ripple for active matrix OLED. Energy Conversion Congress and Exposition (ECCE). Sep. 2011, p. 3229-3233.
Nevertheless, the converter of document (1) is not able to operate as a SIBO-buck for the positive output.
Another interesting converter is proposed in document (2): TEXAS INSTRUMENTS, TPS65136. Single Inductor Multiple Output Regulator for AMOLED. http://www.ti.com/lit/ds/symlink/tps65136.pdf (849 KB). 7 Jul. 2008.
However, the converter of document (2) is not able to operate properly with stability if the load current asymmetry between the negative output current and the positive output current is greater than 30%, meaning that the difference in current at each output needs to stay in close range.
To summarize, the control of a SIBO-Buck-Boost converter adapted to generate required positive output voltage and negative output voltage is not adequately addressed by known techniques.
A possible controller that might be used to solve the above problem is described in document (3) WO2012/032176 wherein it is disclosed a power stage that might be adapted to output a positive output voltage and a negative output voltage. While working satisfactorily, the solution described in document (3) may still be improved to better meet the ever increasing requirements of wireless applications.